Function msQuantizeRasterBuffer

mapquantization.c:129–194

* Compute a palette for the given RGBA rasterBuffer using a median cut quantization. * - rb: the rasterBuffer to quantize * - reqcolors: the desired number of colors the palette should contain. will be set * with the actual number of entries in the computed palette * - forced_palette: entries that should appear in the computed palette * - num_forced_palette_entries: number of entries contai

 *   255, and rb's pixels will have been scaled down to maxsize (see bug #3848)
 */
int msQuantizeRasterBuffer(rasterBufferObj *rb,
      unsigned int *reqcolors, rgbaPixel *palette,
      rgbaPixel *forced_palette, int num_forced_palette_entries,
      unsigned int *palette_scaling_maxval) {
          rgbaPixel **apixels=NULL; /* pointer to the start rows of truecolor pixels */
	
    register rgbaPixel *pP;
    register int col;
    
    unsigned char newmaxval;
    acolorhist_vector achv, acolormap=NULL;
    
    int row;
    int colors;
    int newcolors = 0;
    
    int x;
    /*  int channels;  */
    
	 assert(rb->type == MS_BUFFER_BYTE_RGBA);

    *palette_scaling_maxval = 255;

    apixels=(rgbaPixel**)msSmallMalloc(rb->height*sizeof(rgbaPixel**));
    
    for(row=0;row<rb->height;row++) {
        apixels[row]=(rgbaPixel*)(&(rb->data.rgba.pixels[row * rb->data.rgba.row_step]));
    }

    /*
     ** Step 2: attempt to make a histogram of the colors, unclustered.
     ** If at first we don't succeed, lower maxval to increase color
     ** coherence and try again.  This will eventually terminate, with
     ** maxval at worst 15, since 32^3 is approximately MAXCOLORS.
                  [GRR POSSIBLE BUG:  what about 32^4 ?]
     */
    for ( ; ; ) {
        achv = pam_computeacolorhist(
                apixels, rb->width, rb->height, MAXCOLORS, &colors );
        if ( achv != (acolorhist_vector) 0 )
            break;
        newmaxval = *palette_scaling_maxval / 2;
        for ( row = 0; row < rb->height; ++row )
            for ( col = 0, pP = apixels[row]; col < rb->width; ++col, ++pP )
                PAM_DEPTH( *pP, *pP, *palette_scaling_maxval, newmaxval );
        *palette_scaling_maxval = newmaxval;
    }
    newcolors = MS_MIN(colors, *reqcolors);
    acolormap = mediancut(achv, colors, rb->width*rb->height, *palette_scaling_maxval, newcolors);
    pam_freeacolorhist(achv);
    
    
    *reqcolors = newcolors;


    for (x = 0; x < newcolors; ++x) {
       palette[x].r = acolormap[x].acolor.r;
       palette[x].g = acolormap[x].acolor.g;